The use of liquefied gases and supercritical fluids to carry out extractions has been described in the prior art. Such prior art processes involve the use of supercritical fluids at pressures in excess of 3,000 to 5,000 psi, although in some cases the recommended pressures exceed 10,000 psi.
The prior art processes, however, are generally run as batch processes because of limitations of apparatus and because of limitations involved on materials of construction used to create the apparatus in which the high pressure processes are carried out.
Processes for extractions run at extremely high pressures are described in the following U.S. Pat. Nos. 4,156,688; 4,328,255; 4,466,923; 4,493,854; and 4,495,207.
The prior art has suggested the use of underground reactor vessels and specifically subterranean wells as the situs for carrying out various chemical reactions. Typical of such prior art are the following U.S. Pat. Nos. 3,449,247; 3,464,885; 3,606,999; 3,853,759; and 4,272,383.
These patents, for the most part, disclose the subterranean oxidation of various waste materials under elevated pressure conditions, but do not suggest the extremely high pressures required for the extractions of the type contemplated by the present invention.